a. Technical Field of the Invention
The present invention relates to the measurement of strain in a coated surface and, more particularly, to a method and apparatus which enables such a measurement to be made under realistic thermal and mechanical loads.
b. Description of the Prior Art
Several Standard Test Methods of the American Society for Testing and Materials (ASTM) provide some degree of evaluation of various properties of a coated surface. The most relevant of these methods to the present invention is Test Method D-522 which measures the percent elongation at failure of a coated surface after that surface has been bent 180 degrees around a mandril of known diameter. (It should be noted that "strain" and "percent elongation" are synonymous in this field and the two terms will be used interchangeably in the discussion hereinbelow.) An important characteristic, and, as will appear, disadvantage, of this testing method is that the elastic limit of the substrate material is greatly exceeded at the 180 degree bend.
Another ASTM method, D-2370, measures the strain (percent elongation), at failure, of a free standing sheet of organic coating material. However, the limiting strain in a free-standing sheet is not the same property as the limiting strain of a coated surface.
Other ASTM test methods of background interest include the following: D-4145, which is entitled "Coating Flexibility of Prepared Sheet" and is not a quantitative test; D-3281 which is entitled "Formability of Attached Coatings" and which measures the distance of adhesive failure; D-4146, which is entitled "Formability of Complex Coatings on Steel" and which does not provide a quantitative test; D-4541, which is entitled "Adhesion Pull-Off Strength of Coatings" and which measures the stress perpendicular to the surface and requires an adhesive; D-2197, which is entitled "Adhesion of Organic Coatings by Scrape Adhesion" and which provides a relative test and does not measure strain; and D-3359, which is entitled "Adhesion by Tap Test" and which also provides a relative test and does not measure strain.
In general, the techniques used in the prior art for measuring the strain in a coated surface are designed or intended to work only at room temperature. Further, such techniques are dependent on temperature, the material of the substrate, or both.
More specifically, referring particularly to the ASTM Method D-522, this testing method, as indicated above, does not operate within the elastic limit of the substrate and because engineers do not design structures that operate beyond the elastic limit, it is unrealistic to test a coating by deforming a substrate coated with the coating beyond its elastic limit. Further, the mode of deformation used in this testing method is not real and the interaction of the coating with the substrate is not real, particularly if the substrate cracks open during failure. In addition, the method is very imprecise (insensitive) in making measurements of small strains. Further, the method will not work with brittle substrate materials, such as some of the newer aerospace materials. In addition, the methodology provided under ASTM D-522 does not take the coating through more than one strain cycle.
Other prior art of background interest includes U.S. Pat. Nos. 2,294,897 (Ellis); 2,310,845 (Ellis); 2,724,964 (Singdale); 3,779,065 (Ellis); and 2,645,935 (Pramuk et al). The first four patents relate to methods for determining strain in rigid members wherein a coating of a known maximum strain is applied to a test member, the member is subjected to strain and the crack pattern is observed as the loading is increased. The Pramuk et al patent discloses a method for determining the mechanical properties of cantilever beam of a selected material by loading the beam.